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چکیده
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One distinctive feature of tunnels is the direct interaction between the primary support and the surrounding rocks. For this reason, loads applied to tunnels may vary significantly over time due to changes in rock mass behavior and environmental actions. In this paper, a methodology is proposed for predicting long-term stability of tunnels that accounts for weathering of rock and deterioration of primary support. Chemical weathering of sedimentary rocks around tunnels results in degradation of rock mass shear strength (cohesion and friction angle) and Young’s modulus with time. Two semi-empirical logarithmic equations for modelling of the diminution of rock mass characteristics are presented. Tunnel support systems may also deteriorate and lose their load bearing capacity over time when exposed to aggressive agents. Therefore, main mechanisms of deterioration in common support systems i.e. rock bolts, steel sets and shotcrete are investigated and equations for quantifying damages to their mechanical properties with time are derived. The suggested procedure has been fully illustrated through numerical simulation of a freeway tunnel in Iran (Torshan tunnel) using finite difference method. The tunnel behavior has been evaluated using long-term ground reaction curves (GRC), ground-support interaction diagrams and lining factor of safety FS. Results show that there is a marked increase in the overall ground pressure and rising of internal forces acting on the final lining with time, especially bending moments at the tunnel sidewalls. After 60 years, the lining FS drops below the minimum acceptable value, 1.5 in this case, thus the tunnel rehabilitation is recommended.
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